Vinyl ethers represent a relatively new class of chemistry for preparing photopolymerizable coatings. See for example, J. V. Crivello, J. L. Lee, and D. A. Conlon, Radcure VI Conf. Proc., p. 4-28, Soc. Manuf. Eng., Dearborn, Mich. (1982). The general vinyl ether structure is represented as: EQU CH.sub.2 .dbd.CH--O--R
where R may comprise various types of chemical groups, typically aliphatic or aromatic hydrocarbons. R may include moieties such as ester, urethane, ether, ketone, etc., and/or may comprise siloxanes or fluorocarbons, and there may be more than one vinyl ether group per molecule.
Vinyl ethers can be polymerized by ultraviolet (UV) radiation using either cationic or free radical photoinitiators. When cured cationically, vinyl ethers provide an advantage in increased cure speed over both cationically cured epoxies and free radically cured acrylates. This is related to the electron rich nature of the C.dbd.C double bond in the vinyl ether moiety. In addition, cationic curing of vinyl ethers is not subject to oxygen inhibition as is free radical cure of acrylates, where cure speed decreases in the absence of inert gas blanketing. When cured free radically, vinyl ethers do not homopolymerize well, but may be copolymerized with other components such as acrylates or unsaturated esters. See J. R. Snyder et al, "Free-radical Co-polymerization of Acrylates and Vinyl Ethers", U.S. Pat. No. 5,352,713; J. Gaske et al, "Method of Coating Concrete Floors With Photocurable Coatings", U.S. Pat. No. 4,999,216; G. K. Noren et al, "Free-radical Curable Compositions", U.S. Pat. No. 5,334,456. Curing in such hybrid systems may also be carried out by simultaneous free radical and cationic photo polymerization. An anticipated advantage of most vinyl ethers over acrylates is a lower level of skin sensitization. A further benefit of vinyl ethers is increased UV transparency, especially for applications in which optical fiber gratings are written into fiber cores with laser radiation through the optical fiber coating. See J. Aspell, D. Inniss, V. J. Kuck, M. A. Paczkowski, D. A. Simoff, "Formation of Gratings in Polymer Coated Optical Fibers", U.S. Pat. No. 5,620,495. A still further potential benefit of cationically cured fiber coating systems is enhanced corrosion resistance and mechanical strength resulting from an acidic environment. See J. R. Petisce, "Optical Fiber Including Acidic Coating System", U.S. Pat. No. 5,181,269.
Many UV curable coatings contain at least one oligomer component, together with one or more reactive monomer(s) and photoinitiator(s). An increasing variety of vinyl ether monomers are commercially available. Typical synthesis methods proceed via acetylene chemistry, dehydrohalogenation, cracking of acetals, and catalytic transvinylation. See for example, J. W. Reppe, Acetylene Chemistry, Charles A. Meyer, New York (1949); D. H. Lorenz and N. B. Bikales, Encycl. Polym. Sci., Wiley-Interscience, New York (1964); Matzher, Kurkjy and Cotter, Chem. Rev., 64, 645 (1964); C. Y. Yang, Ph. D. Thesis, 1991, Rennselear Polytechnic Institute, Troy, N.Y. 12180; M. Dimonie, Teodorscu, Makromol. Chem. Rapid. Commun., 14, 303 (1993); S. -S. Thang, Q. -G Liu and L. -L. Yang, J. Polym. Sci., Part A Polym. Chem., 31, 2313 (1983); J. V. Crivello, K. -D Jo, J. Polym Sci., Part A, 31, 1473 (1993); J. E. McKeon et al, Tetrahedron, 28 223 (1972); M. A. Smith et al, Polymer Reprints, 28(2), 264 (1987). Presently, only a few vinyl ether functional oligomers are commercially available. These typically contain urethane and/or ester groups which are undesirable for some applications. See S. C. Lapin, RadTech '88 Conf. Proc., p. 395, RadTech International, Northbrook, Ill. (1988); S. C. Lapin et al, "Vinyl Ether Terminated Urethane Resins", U.S. Pat. No. 4,751,273. Urethane and ester groups absorb UV light which is detrimental for coatings designed for UV transparency. A further disadvantage of ester and urethane groups is that they tend to decrease cure speed in cationically cured systems, limiting the inherent potential of vinyl ethers for rapid cure. To compensate for the impedance of cure speed by these constituent groups the concentration of the photoinitiator may be increased. However, this adds cost, and is detrimental to UV transparency of the coating because photoinitiators are UV absorbent by design.
Oligomers or polymers that are not vinyl ether functionalized have been used in formulations with vinyl ether monomers. Such oligomers/polymers have included non-reactive resin fillers such as cellulose derivatives or poly(alkyl methacrylates). See S. C. Lapin, "Semi-interpenetrating Polymer Networks", U.S. Pat. No. 4,654,379; E. J. Murphy et al, "Radiation Curable Coating Composition and Coated Optical Fiber", U.S. Pat. No. 5,596,669. Other additives of the prior art are reactive acrylate-functional oligomers, unsaturated polyester oligomers, or epoxy-functional oligomers. See for example C. E. Bayha, "Cationically Initiated Curable Resin System, U.S. Pat. No. 5,252,682; J. A. Dougherty et al, "Vinyl Ethers for Cationic UV Curing", Radcure '86 Conf. Proc., 15-1, Soc. Manuf. Eng., Dearborn, Mich. (1986); J. A. Dougherty et al, "Triethylene Glycol Divinyl Ether as a Reactive Diluent for Cationic Curing", Radcure Europe '87 Conf. Proc., 5-1, Soc. Manuf. Eng., Dearborn, Mich. (1987). Many of these oligomers/polymers contain urethane and/or ester groups, with attendant disadvantages in UV transparency and cure speed. The epoxy-functional oligomers that are commercially available (e.g., the diglycidyl ether of Bisphenol A or its derivatives) often contain aromatic groups which are unacceptable for UV transparency and also increase the tendency toward yellowing. In general, the commercially available cycloaliphatic (non-aromatic) epoxies do not have sufficiently high molecular weight to provide adequate viscosity for optical fiber coating applications.
Earlier patents for vinyl ether based optical fiber coatings tend to rely on oligomers that contain ester and/or urethane groups. For example, U.S. Pat. No. 4,956,198 discloses vinyl ether functional oligomers having aromatic urethane moieties. U.S. Pat. No. 5,139,872 discloses vinyl ether based coatings for optical fibers wherein the oligomer is a vinyl ether urethane having a polyester or polyether backbone. U.S. Pat. No. 5,352,712 discloses hydrocarbon polyol based aliphatic vinyl ether urethane oligomers for use in optical fiber coatings.
Aliphatic polyether and aliphatic hydrocarbon based oligomers have long been used in fiber optic coatings, where oligomers having such backbones provide a combination of desirable properties including flexibility, low glass transition temperatures, and resistance to hydrolysis. A particularly desirable backbone is polytetrahydrofuran, which has superior hydrophobicity compared to other polyethers such as polyethylene oxide and polypropylene oxide, and has superior oil resistance as compared with saturated hydrocarbon backbones. Polytetrahydrofuran also has excellent UV transparency at wavelengths above about 240 nm, where it is desirable to be able to write optical fiber gratings through the polymer coating. Polytetrahydrofuran is also known by other names, e.g., poly(tetramethylene oxide), polybutanediol, poly(butylene glycol), and .alpha.-hydroxy-.omega.-hydroxy poly(oxy-1,4-butanediyl).
The term oligomer as used herein, and generally as used in the art, refers to relatively low molecular weight materials, e.g. with molecular weights in the approximate range 500-5000. The distinction between oligomer to polymer in the art is not well defined but the range given is suitable for this description.
A technique for preparing polytetrahydrofuran (PTHF) vinyl ethers is described in U.S. Pat. No. 4,967,015. This process reacts polybutanediol with acetylene in the presence of a strong base. However, in commercial practice this process has been limited to synthesis of relatively low molecular weight oligomers, which have viscosities that are too low for commercial optical fiber coating techniques.
It would represent a substantial contribution to the technology if high molecular weight vinyl ether functionalized oligomers or polymers became available which are substantially devoid of aromatic moieties, carbonyl groups and nitrogen, i.e. species that are detrimental to both UV transparency and fiber coating cationic cure speed. Desirable types of oligomer or polymer backbones that meet these requirements include aliphatic polyethers, cycloaliphatic polyethers, aliphatic hydrocarbons, cycloaliphatic hydrocarbons, and their copolymers and mixtures. Other desirable backbones include aliphatic or cycloaliphatic hydrocarbon or polyether polyols that have been chain extended with aliphatic or cycloaliphatic di- or multifunctional epoxides. The availability of high molecular weight polytetrahydrofuran based vinyl ether functional oligomers would be of special interest in optical fiber coating technology.